422 BULLETIN OF THE UNITHl) S'lWTKS FJSII COMMISSION. 



three filaments (/j7) in a region wlieie the projections oceur. The conical points, which 

 I would designate as ciliated spurs {cs), protrude abruptly into the water tube between 

 the lamellae of the gill. 



A section passing transversely through the filaments, and in the long axis of a 

 few of these ciliated spurs, is shown in Fig. 86, at cs. Their walls are seen to be 

 merely a continuation of the single-cell-layered wall of the filament. They are closely 

 interlocked by cilia. Their intericn- is filled up with a solid mass of cells whose nature 

 1 have not positively determined. In macerated specimens, they appear to be much 

 elongated in the long axis of the spiu', but sections failed to show this satisfactorily. 

 These projecting s])urs are very conspicuous in any macerated preparation of the gill 

 of Pecten. 



GILL OF VENIS MERCENARIA. 



This folded gill is, in many Avays, more complicated than that of Pect en. The fila- 

 ments have a vascular connection with one another at their inner edges, spaces being- 

 left here and there to allow water to enter the water tube of the gill (Fig. 70, u\ PI. 

 Lxxxviii). The great primary folds, marked off by the i^rimary reentering angles (/•«]), 

 include about seventy filaments, Tlu^se folds are sometimes divided, however, by 

 a second reentering angle (ra^), into two secondary folds. There is no comiection 

 between the gill lamellae at this point. At the primary reentering* angle there is a 

 partition between lamellae, consisting principally of muscle bundles {mus). Between 

 these exists a blood channel (6r). From these channels is sometimes given oif into 

 the water tube on either side of the partition a huge blood sinus (jybs). These are 

 so large as to almost completely fill the space between the lamellte. 



Similar blood siiuises appear in other cases irom an enlargement of the filament 

 at the img]efbs, and may also be so great as to entirely fill the water tube. In re- 

 gions where this condition is present, blood sinuses from the partitions are very small 

 or absent. 



Venus being a large form and quite active, must require a considerable amount 

 of food and at the same time sufficient aeration of blood. The narrow filaments, not 

 snfSciently large to contain much blood, seem to have been specialized for ])r(>curing 

 food. These large blood sinuses may have been developed to provide for the dimin- 

 ished aeration in the filaments. Though they nearly fill the water tube, their thin 

 walls are surrounded by water on all sides, probably in (]uantities sufiicient for the 

 l)ur])ose. 



THE (JILL OF OSTREA VIRGINIAXA. 



The gill of this very degenerate form is probably the most complex in the group. 

 The lamellae are thrown into a number of folds between each of the thick cross- 

 ])artitions. The filaments may be seen ahnost everywhere in section (Fig. 70, PI. xc) 

 U) have a vascular connection with one another at their inner edges. This is the case 

 w ith all the filaments in the fold marked 2 in the figure to which reference has been 

 made. Here the entire inner space of the fold is a blood sinus, with which the blood 

 channel of every filament is continuous. In fold 1, three filaments at its outer extrem- 

 ity are connected by their inner edges. In fold 3, four filaments on the left are thus 

 connected, and this common sinus is seen to be continuous with another, close under 

 the wall of tlie water tube of the gill {'ivt). 



